1. Field of the Invention
The present invention concerns a mixed powder for powder metallurgy based on a Fe powder and an alloy powder and capable of providing a sintered product having high density and high strength and with less scattering in dimensional accuracy upon sintering, as well as a sintered product obtained therefrom.
2. Description of the Prior Art
Powder metallurgy is a process of compacting and then sintering a metal powder as a raw material into a final product which drastically changes existent production processes comprising, for example, rolling, forging and casting. Accordingly, it is possible by the powder metallurgy to produce parts which were difficult to be produced by existing melting processes, for example, high melting metal materials such as W and Mo, porous materials for oil-impregnated bearings or filters, super hard alloys or thermits. In addition, since the powder metallurgy has various kinds of advantages not obtainable with materials produced by melting, for example, a merit from a view point of production such as improved material yield attained by non-cutting and high dimensional accuracy and a merit from, a view point of physical properties such as less segregation and anisotropy which are liable to occur in the materials produced by melting. Accordingly, various kinds of parts, which were produced so far by the melting process have now been produced also by powder metallurgy.
Most of sintered products produced at present by the powder metallurgy are used for automobile parts and, among all, sintered Fe materials have been used generally. Various sintered Fe materials have been known and, for example, materials prepared by mixing fine powders of graphite, copper, etc. to a Fe powder as the main ingredient and sintering them with an aim of improving strength, weather proofness, abrasion resistance, etc. have been known. Further, with a view point of extending the application range of sintering parts, higher toughness and strength have become demanded for the sintered parts and a method of adding and alloying elements such as Ni and Mo has also been known as a means for achieving such a demand.
By the way, as a typical method for obtaining a high strength Fe series sintered product by the powder metallurgy, a premix method and a prealloying method has been known.
Premixing is a method of homogeneously mixing a Fe powder with a metal powder or an alloy powder (hereinafter some time referred to as added metal powder), compacting them and subsequently sintering them under heating to solid-solubilize added elements. The method has a merit that the molding fabrication is relatively simple but it involves a drawback that the added metal powder in the Fe powder causes separation or segregation due to difference of the specific gravity in the course up to compaction or diffusion of the added metal powder does not proceed sufficiently during sintering, which leads to a quality problem of causing scattering in the strength and the size of the sintered product.
On the other hand, the prealloying is a method of using an alloyed steel powder in which alloying elements such as Ni, Cu, Mo and Cr are previously solid-solubilized in Fe and the method is free from the problem as mentioned for the premixing method. However, since the alloyed steel powder obtained by prealloying is extremely hard as compared with a pure Fe powder, it involves a problem that compaction density can not be increased sufficiently during compaction making it difficult to obtain a sintered product of high density and, accordingly, physical properties of the alloyed steels can not be enjoyed sufficiently.
Each of the methods described above has respective merits and demerits, but it is considered that the premixing is more advantageous than the prealloying in obtaining a desired sintered product if the above-mentioned disadvantage such as occurrence of segregation or insufficient diffusion can be overcome.
By the way, for the method of preventing the segregation, a method of depositing a graphite powder on an iron-steel powder by using an organic binder has been proposed as described, for example, in Japanese Patent Laid-Open Sho 56-136901 and Sho 63-103001. Further, a so-called diffusing deposition method of diffusing to deposit other metal or alloy powder to a Fe powder has been developed as described, for example, in Japanese Patent Publication Sho 45-9649 and Japanese Patent Laid-Open Sho 63-297502. Particularly, the diffusing deposition method scarcely degrades a compacting property and can prevent the problem of scattering in the strength reduction and the dimensional accuracy caused by the segregation to some extent. That is, in the diffusing deposition type alloyed steel powder, an elemental metal powder of Ni, Cu, Mo, etc., or an alloyed powder thereof is added to and uniformly mixed with a Fe powder and the added metal powder is diffused to deposit to the surface of the Fe powder by a diffusing treatment, in which the powder once deposited by diffusion causes no segregation.
On the other hand, various technics have been proposed so far also for improving the diffusibility, but most of the existent technics have been based on the view point considering the kind and the amount of the added metal powder and none of them mentions to the sintering behavior which is expected to provide a significant effect on the appearance of the strength.